High Stability, Self-Protecting Electrocatalyst Particles

ABSTRACT

High-stability, self-protecting particles encapsulated by a thin film of a catalytically active noble metal are described. The particles are preferably nanoparticles comprising a passivating element having at least one metal selected from the group consisting of columns IVB, VB, VIB, and VIIB of the periodic table. The nanoparticle is preferably encapsulated by a Pt shell and may be either a nanoparticle alloy or a core-shell nanoparticle. The nanoparticle alloys preferably have a core comprised of a passivating component alloyed with at least one other transition metal. The core-shell nanoparticles comprise a core of a non-noble metal surrounded by a shell of a noble metal. The material constituting the core, shell, or both the core and shell may be alloyed with one or more passivating elements. The self-protecting particles are ideal for use in corrosive environments where they exhibit improved stability compared to conventional electrocatalyst particles.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119(e) to U.S.Provisional Patent Application No. 61/155,196 which was filed on Feb.25, 2009, the entirety of which is incorporated by reference as if fullyset forth in this specification.

STATEMENT OF GOVERNMENT RIGHTS

This invention was made with Government support under contract numberDE-AC02-98CH10886 awarded by the U.S. Department of Energy. TheGovernment has certain rights in the invention.

BACKGROUND

I. Field of the Invention

This invention relates generally to self-protecting electrocatalystparticles. The present invention also relates to the controlleddeposition of a catalytically active metal film on high-stability,self-protecting electrocatalyst particles. This invention furtherrelates to the use of these electrocatalyst particles in energyconversion devices such as fuel cells.

II. Background of the Related Art

A fuel cell is an electrochemical device capable of converting thechemical energy of a fuel and an oxidant into electrical energy. Astandard fuel cell is comprised of an anode and cathode separated by aconducting electrolyte which electrically insulates the electrodes yetpermits the flow of ions between them. The fuel cell operates byseparating electrons and ions from the fuel at the anode andtransporting the electrons through an external circuit to the cathode.The ions are concurrently transported through the electrolyte to thecathode where the oxidant is combined with the ions and electrons toform a waste product. An electrical circuit is completed by theconcomitant flow of ions from the anode to cathode via the conductingelectrolyte and the flow of electrons from the anode to the cathode viathe external circuit.

The science and technology of fuel cells has received considerableattention, being the subject of numerous books and journal articlesincluding, for example, “Fuel Cells and Their Applications,” by K.Kordesch and G. Simader, New York, N.Y.: VCH Publishers, Inc. (2001).Although there are various types of fuels and oxidants which may beused, the most significant is the hydrogen-oxygen system. In ahydrogen-oxygen fuel cell, hydrogen (H₂) is supplied to the anode as thefuel where it dissociates into H⁺ ions and provides electrons to theexternal circuit. Oxygen (O₂) supplied to the cathode undergoes areduction reaction in which O₂ combines with electrons from the externalcircuit and ions in the electrolyte to form H₂O as a byproduct. Theoverall reaction pathways leading to oxidation at the anode andreduction at the cathode are strongly dependent on the materials used asthe electrodes and the type of electrolyte.

Under standard operating conditions the H₂ and O₂ oxidation/reductionreactions proceed very slowly, if at all, requiring elevatedtemperatures and/or high electrode potentials to proceed. Reactionkinetics at the electrodes may be accelerated by the use of noble metalssuch as platinum (Pt), palladium (Pd), ruthenium (Ru), and relatedalloys. Electrodes formed of these materials function aselectrocatalysts since they accelerate electrochemical reactions atelectrode surfaces yet are not themselves consumed by the overallreaction. Further improvements have been attained by incorporating noblemetal-containing particles or structures with reduced dimensions. Areduction to nanoscale dimensions yields a significant increase in thesurface-to-volume ratio, producing a concomitant increase in the surfacearea available for reaction. Despite the performance improvementsattainable with nanoscale electrocatalysts, successful commercializationof fuel cells requires still further increases in performance,stability, and cost efficiency.

Pt has been shown to be one of the best electrocatalysts, but itssuccessful implementation in commercially available fuel cells ishindered by its high cost, susceptibility to carbon monoxide (CO)poisoning, poor stability under cyclic loading, and the relatively slowkinetics of O₂ reduction at the cathode. A variety of approaches havebeen employed in attempting to solve these problems. An example is U.S.Pat. No. 6,232,264 to Lukehart, et al. which discloses polymetallicnanoparticles such as platinum-palladium alloy nanoparticles for use asfuel cell electrocatalysts. Another example is U.S. Pat. No. 6,670,301to Adzic, et al. which discloses a process for depositing a thin film ofPt on dispersed Ru nanoparticles supported on carbon (C) substrates.These approaches have resulted in electrocatalysts with reduced Ptloading and a higher tolerance for CO poisoning. Both of theaforementioned patents are incorporated by reference as if fully setforth in this specification.

Attempts to accelerate the oxygen reduction reaction (ORR) on Pt whilesimultaneously reducing Pt loading have been met with limited success.Recent approaches have utilized high surface area Pt or Pd nanoparticlessupported by nanostructured carbon (Pt/C or Pd/C) as described, forexample, in U.S. Pat. No. 6,815,391 to Xing, et al., which isincorporated by reference as if fully set forth in this specification.However, as an oxygen reduction catalyst, bulk Pt is still several timesmore active than Pt/C and Pd/C nanoparticle electrocatalysts. Anotherapproach involves the use of Pt-encapsulated core-shell or alloynanoparticles as described, for example, in U.S. Patent Publ. No.2007/0031722 to Adzic, et al., which is incorporated by reference as iffully set forth in this specification. The quantity of noble metalrequired was reduced even further by using a core-shell nanoparticlewith a noble metal shell, but a non-noble metal core. Despite thecontinued improvement attained with Pt-based electrocatalysts,successful implementation in commercial energy conversion devices suchas fuel cells requires still further increases in the catalytic activitywhile simultaneously improving long-term stability and reducing theamount of costly precious metals required.

SUMMARY

In view of the above-described problems, needs, and goals, in oneembodiment of the present invention a self-protecting electrocatalystwith high stability is provided. It has been discovered that non-noblemetal particles may exhibit reduced stability when used with corrosivematerials such as may be found in an energy conversion device. If thenon-noble metal component is not completely encapsulated by a noblemetal, the non-noble metal may be subject to dissolution in a corrosiveenvironment. This may occur, for example, due to exposure to the acidicor alkaline solution comprising the electrolyte in a fuel cell. In oneembodiment, this problem is solved by synthesizing high-stability,self-protecting electro catalysts supports with a component that easilypassivates. An example of a support that passivates is a binary metalalloy. This may be expressed as Tr_(1-x)Ps_(x) where Tr is a transitionmetal and Ps is a passivating metal. The individual amounts (x) and(1−x) of each element may be adjusted over the range 0<x<1 to obtain acompound with the desired structure, phase, and properties as maygenerally be determined from a binary alloy phase diagram of theconstituent elements. The passivating metal is preferably an elementfrom column IVB, VB, VIB, or VIIB of the periodic table which correspondto groups 4 through 7, respectively. The passivating component(s)therefore preferably comprise the metals titanium (Ti), hafnium (Hf),zirconium (Zr), tungsten (W), tantalum (Ta), niobium (Nb), vanadium (V),rhenium (Re), molybdenum (Mo), technetium (Tc), chromium (Cr), andmanganese (Mn). One or a combination of the aforementioned metals may beused along with the primary materials constituting the electrocatalyst.

The electrocatalyst is preferably a particle having dimensions rangingfrom 1-100 nm and is thus a nanoparticle, but is not so limited.Particles extending into the micrometer or even millimeter size rangemay also be used. The electrocatalyst preferably comprises ananoparticle covered by a contiguous thin layer of a catalyticallyactive noble metal with a surface coverage ranging from less than amonolayer (ML) to several MLs. The catalytically active noble metal canbe Ru, rhodium (Rh), Pd, osmium (Os), iridium (Ir), Pt, or gold (Au),but is preferably Pt. In an especially preferred embodiment, theelectrocatalyst is a nanoparticle having a Pt surface coverage of oneML. The nanoparticle itself may comprise either a nanoparticle alloy ora core-shell nanoparticle which is covered with a catalytically activenoble metal. The electrocatalyst is therefore preferably either aPt-coated nanoparticle alloy or a Pt-coated core-shell nanoparticle.

In one embodiment, the nanoparticle comprises at least one element fromcolumn IVB, VB, VIB, or VIIB of the periodic table which is alloyed withone or more other transition metals. The thus-formed alloy is preferablyhomogeneous, but may have compositional and structural nonuniformities.The passivating component is preferably present in a minimumconcentration sufficient to passivate exposed non-noble metal coresurfaces and inhibit corrosion of the nanoparticle alloy core. In thisembodiment the electrocatalyst is preferably a Pt-coated nanoparticlealloy core in which the core is a homogeneous solid solution comprisingat least one element from column IVB, VB, VIB, or VIIB of the periodictable.

In another embodiment, the nanoparticle is a core-shell nanoparticle inwhich a core comprising one or more element from column IVB, VB, VIB, orVIIB of the periodic table is alloyed with one or more other transitionmetals and is encapsulated by a shell of one or more noble metals. Thenoble metal shell is atomically thin, i.e. less than one to several MLsthick. The noble metal shell is preferably at least a ML thick, but isnot so limited and may be several atomic layers. The composition of thecore itself is preferably homogeneous, but may be nonuniform. Thepassivating component is preferably present in a minimum concentrationsufficient to passivate exposed non-noble metal regions of core surfaceand thus inhibit corrosion of the underlying core, but is not solimited. In this embodiment the catalyst is preferably a Pt-coatedcore-shell nanoparticle in which the core is a homogeneous alloycomprising at least one element from column IVB, VB, VIB, or VIIB of theperiodic table and the shell is a monolayer of a noble metal.

In still another embodiment, the particle is a core-shell nanoparticlein which a core comprising a transition metal is encapsulated by a shellof one or more noble metals alloyed with one or more elements fromcolumns IVB, VB, VIB, or VIIB of the periodic table. The noble metalshell is preferably at least a ML thick, but may be several MLs. Thecomposition of the shell is preferably homogeneous, but may benonuniform. The passivating component is preferably present in a minimumconcentration sufficient to passivate the core surface and thus inhibitcorrosion of the underlying core, but is not so limited. In thisembodiment the catalyst is preferably a Pt-coated core-shellnanoparticle in which the core is a transition metal and the shell is aML of at least one noble metal which is preferably Pd either alone oralloyed with at least one element from column IVB, VB, VIB, or VIIB ofthe periodic table. This interlayer of Pd makes the core or core-shellsurface suitable for interacting with Pt to promote its activity for theoxidation reduction reaction (ORR).

In yet another embodiment, the particle is a core-shell nanoparticle inwhich both the core and shell are separately alloyed with one or moreelements from columns IVB, VB, VIB, or VIIB of the periodic table. Thecore comprises at least one transition metal alloyed with at least onepassivating element whereas the shell is at least one noble metalsimilarly alloyed with at least one passivating element. In this mannerthe passivating component is present in both the core and the shell of acore-shell nanoparticle.

An additional embodiment relates to the utilization of thesehigh-stability, self-protecting Pt-coated nanoparticle alloys orPt-coated core-shell nanoparticles in the electrodes of a fuel cell. Ina preferred embodiment, the self-protecting Pt-coated nanoparticleelectrodes are used as the cathode to accelerate ORR kinetics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross-sectional schematic of a nanoparticle alloy in whichthe core comprises a homogeneous alloy having a passivating element;

FIG. 1B shows a cross-sectional schematic of a core-shell nanoparticlein which the core comprises a homogeneous alloy having a passivatingelement and the shell is a noble metal;

FIG. 1C is a cross-sectional schematic of a core-shell nanoparticle inwhich the core comprises a transition metal and the shell is a noblemetal alloyed with a passivating element;

FIG. 1D is a cross-sectional schematic of a core-shell nanoparticle inwhich both the core and the shell are alloyed with a passivatingelement;

FIG. 2 shows a series of images illustrating the underpotentialdeposition of a Cu adlayer onto a nanoparticle followed by the galvanicdisplacement of Cu atoms by Pt; and

FIG. 3 is an illustration of a Pt-encapsulated nanoparticle alloy inwhich the core comprises a homogeneous alloy with a passivating element;

FIG. 4 is an illustration of a Pt-encapsulated nanoparticle with aninterlayer of Pd or another noble metal in which the core comprises ahomogeneous alloy with a passivating element;

FIG. 5 is a transmission electron microscopy image of carbon-supportedPd₃Ti nanoparticles (Pd₃Ti/C) showing a typical structure and sizedistribution;

FIG. 6 shows a plot of the intensity of the diffracted X-ray signal as afunction of 2θ for Pd₃Ti/C and Pd/C nanoparticles;

FIG. 7 is a plot showing a comparison of polarization curves for oxygenreduction on Pt/C (left curve) and Pt/Pd₃Ti/C (right curve)nanoparticles;

FIG. 8 shows a bar graph comparing the Pt mass-specific activities ofcommercial Pt/C nanoparticles and Pt/Pd₃Ti/C nanoparticles expressed asthe current j_(k) in mA/μg at 0.90 V;

FIG. 9 is a plot comparing X-ray diffraction spectra obtained for PdRe/Cannealed in a H₂ ambient at 800° C. for 3 hours (top curve) and at 600°C. for 3 hours (middle curve) with commercial Pd/C nanoparticles;

FIG. 10 shows a bar graph comparing the Pt mass-specific activities ofcommercial Pt/C nanoparticles and Pt/PdRe/C nanoparticles expressed asthe current j_(k) in mA/μg at 0.90 V;

FIG. 11 is a plot comparing polarization curves for oxygen reduction onPt/PdRe/C nanoparticles before (top curve) and after (bottom curve)10,000 potential cycles; and

FIG. 12 is a schematic showing the principles of operation of a fuelcell in which at least one electrode may be comprised of Pt-encapsulatedcore-shell nanoparticles, according to the present invention.

DETAILED DESCRIPTION

These and other objectives of the invention will become more apparentfrom the following description and illustrative embodiments which aredescribed in detail with reference to the accompanying drawings. In theinterest of clarity, in describing the present invention, the followingterms and acronyms are defined as provided below.

ACRONYMS

-   -   AES: Auger Electron Spectroscopy    -   ALD: Atomic Layer Deposition    -   CVD: Chemical Vapor Deposition    -   GC: Glassy Carbon    -   MBE: Molecular Beam Epitaxy    -   ML: Monolayer    -   ORR: Oxidation Reduction Reaction    -   PLD: Pulsed Laser Deposition    -   PVD: Physical Vapor Deposition    -   TEM: Transmission Electron Microscope    -   UPD: Underpotential Deposition    -   XRD: X-ray Diffraction

DEFINITIONS

-   -   Adatom: An atom located on the surface of an underlying        substrate.    -   Adlayer: A layer (of atoms or molecules) adsorbed to the surface        of a substrate.    -   Atomically Thin Having a thickness of less than one to several        monolayers.    -   Catalysis: A process by which the rate of a chemical reaction is        increased by means of a substance (a catalyst) which is not        itself consumed by the reaction.    -   Electrocatalysis: The process of catalyzing a half cell reaction        at an electrode surface.    -   Electrodeposition: Another term for electroplating.    -   Electrolyte: A substance comprising free ions which behaves as        an electrically conductive medium.    -   Electroplating: The process of using an electrical current to        reduce cations of a desired material from solution to coat a        conductive substrate with a thin layer of the material.    -   Monolayer: A single layer (of atoms or molecules) which occupies        available surface sites and covers substantially the entire        exposed surface of a substrate.    -   Multilayer: More than one layer (of atoms or molecules) on the        surface, with each layer being sequentially stacked on top of        the preceding layer.    -   Nanoparticle: Any manufactured structure or particle with        nanometer-scale dimensions (i.e., 1-100 nm).    -   Noble metal: Metals which are extremely stable and inert, being        resistant to corrosion or oxidation. These generally comprise        ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag),        rhenium (Re), osmium (Os), iridium (Ir), platinum (Pt), and gold        (Au). Noble metals are frequently used as a passivating layer.    -   Non-noble metal: A metal which is not a noble metal.    -   Redox reaction: A chemical reaction in which an atom undergoes a        change in oxidation number. This typically involves the loss of        electrons by one entity accompanied by the gain of electrons by        another entity.    -   Refractory metal: A class of metals with extraordinary        resistance to heat and wear, but with generally poor resistance        to oxidation and corrosion. These generally comprise tungsten        (W), molybdenum (Mo), niobium (Nb), tantalum (Ta), and rhenium        (Re).    -   Submonolayer: Surface (atom or molecular) coverages which are        less than a monolayer.    -   Transition metal: Any element in the d-block of the periodic        table which includes groups 3 to 12 and IB through VIIIB.        Columns 4 through 7 correspond to columns IVB through VIIB,        respectively.

The present invention is based on the realization that unwanteddissolution of electrocatalyst particles may be inhibited by inclusionof a material which easily passivates. The passivating element ispreferably an element from columns IVB through VIIB of the periodictable (corresponding to groups 4 through 7, respectively) and is suchthat it forms a stable chemical bond with elements or compounds foundwithin the corrosive environment. This produces non-reactive surfaceregions which inhibit corrosion of the underlying material constitutingthe bulk of the particle. This is particularly useful during, forexample, application of high electric potentials and exposure to highlycorrosive environments. It has been shown that the inclusion of anon-noble metal core in particle alloys and/or core-shell particlesreduces the amount of more costly noble metals such as Pt and Pdrequired while simultaneously providing a large surface area of the morecatalytically active noble metal. However, the inadvertent formation ofan incomplete noble metal coating or shell may produce pinholes whichleave the core constituents exposed to what is typically a corrosiveenvironment. This may inevitably lead to the subsequent dissolution ofthe non-noble metal component and a degradation of the catalyticproperties of the particles. As disclosed in detail below, the inclusionof a passivating component produces high-stability, self-protectingparticle alloys and/or core-shell particle electrocatalysts.

The particles disclosed and described in this specification are notlimited to any particular shape or size, but are preferablynanoparticles with sizes ranging from 1 to 100 nm in one moredimensions. However, the size is not so limited and may extend into themicrometer and millimeter size range. The shape is preferably sphericalor spheroidal, but again is not so limited. Throughout thisspecification, the particles will be primarily disclosed and describedas essentially spherical nanoparticles. It is to be understood, however,that the particles may take on any shape, size, and structure as iswell-known in the art. This includes, but is not limited to branching,conical, pyramidal, cubical, mesh, fiber, cuboctahedral, and tubularnanoparticles. The nanoparticles may be agglomerated or dispersed,formed into ordered arrays, fabricated into an interconnected meshstructure, either formed on a supporting medium or suspended in asolution, and may have even or uneven size distributions. The particleshape and size is preferably configured so that the bondingconfiguration of surface atoms is such that their reactivity and, hence,their ability to function as a catalyst is increased.

I. Nanoparticle Synthesis

Nanometer-scale particles or nanoparticles have been formed from a widevariety of materials using a number of different techniques whichinvolve both top-down and bottom-up approaches. Examples of the formerinclude standard photolithography techniques, dip-pen nanolithography,and focused ion-beam etching. The latter comprises techniques such aselectrodeposition or electroplating on templated substrates, laserablation of a suitable target, vapor-liquid-solid growth of nanowires,and growth of surface nanostructures by sputtering, chemical vapordeposition (CVD) or molecular beam epitaxy (MBE) from suitable gasprecursors and/or solid sources.

Nanoparticles may also be formed using conventional powder-processingtechniques such as comminution, grinding, or chemical reactions.Examples of these processes include mechanical grinding in a ball mill,atomization of molten metal forced through an orifice at high velocity,centrifugal disintegration, sol-gel processing, or vaporization of aliquefied metal followed by supercooling in an inert gas stream.Powder-processing techniques are advantageous in that they are generallycapable of producing large quantities of nanometer-scale particles withdesired size distributions. Chemical routes involve solution-phasegrowth in which, as an example, sodium boronhydride, superhydride,hydrazine, or citrates may be used to reduce an aqueous or nonaqueoussolution comprising salts of a non noble metal and noble metal.Alternatively, the salt mixtures may be reduced using H₂ gas attemperatures ranging from 150 to 1,000° C. These chemical reductivemethods can be used, for example, to make nanoparticles of palladium(Pd), gold (Au), rhodium (Rh), iridium (Ir), ruthenium (Ru), osmium(Os), rhenium (Re), nickel (Ni), cobalt (Co), iron (Fe), andcombinations thereof. In this specification, two primary types ofself-protecting nanoparticles will be discussed and described in detail.The first is nanoparticle alloys while the second is core-shellnanoparticles.

A. Nanoparticle Alloys

A nanoparticle alloy is generally defined throughout this specificationas a nanoparticle comprised of a complete solid solution of two or moreelemental metals. The alloy comprises at least one noble metal and atleast one element which easily passivates in acidic or alkalinesolutions. The relative concentration of each element is dependent onthe particular application and desired properties, but is preferablysuch that a minimum quantity of the passivating element sufficient toform a surface barrier is present in the alloy. The passivating elementpreferably comprises one or more elements selected from column IVB, VB,VIB, or VIIB (corresponding to groups 4 through 7, respectively) of theperiodic table. The passivating element therefore comprises at least oneof titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), zirconium(Zr), niobium (Nb), molybdenum (Mo), technetium (Tc), hafnium (Hf),tantalum (Ta), tungsten (W), and rhenium (Re). These elements have apropensity for forming a stable bond with one or more constituents ofacidic or alkaline solutions.

By forming an alloy between one or more passivating metals and theelements comprising the nanoparticle, an electrocatalyst with improvedstability may be obtained. Generally, a binary metal alloy comprising atransition metal and a passivating metal may be expressed asTr_(1-x)Ps_(x) where Tr is the transition metal and Ps is thepassivating metal. The individual amounts (x) and (1−x) of each elementmay be adjusted over the range 0<x<1 to obtain a compound with thedesired structure, phase, and properties as may be determined from abinary alloy phase diagram of the constituent elements. Examples ofnoble metal/passivating metal binary alloys include, but are not limitedto Pd_(1-X)Ti_(x), Pd_(1-x)W_(x), Pd_(1-x)Nb_(x), Pd_(1-x)Ta_(x),Pd_(1-x)Re_(x), Pd_(1-x)Ir_(x), Ir_(1-x)Ti_(x), Ir_(1-x)Ta_(x),Ir_(1-x)Nb_(x), Ir_(1-x)Re_(x), Au_(1-x)Ta_(x), Au_(1-x)Ir_(x), andAu_(1-x)Re_(x). Alloys comprising Re as the valve metal are especiallypreferred since Re is relatively stable and it exhibits excellentsolubility with a large number of transition metals, particularly othernoble metals.

In another embodiment the alloy may comprise one or more of a noblemetal, a non-noble metal, and a passivating metal to form a ternaryalloy as shown, for example, in FIG. 1A. In this embodiment thenanoparticle alloy is comprised of a solid solution of a noble metal(1), a non-noble metal (2), and a passivating metal (3). The inclusionof a non-noble metal (2) reduces the amount of more costly noble metals(1) which may be required. The non-noble metal may be any transitionmetal other than the noble metal or passivating metal used in thenanoparticle alloy. A transition metal is defined as any metal withinthe d-block of the periodic table which corresponds to groups threethrough twelve. The relative quantities of the non-noble metal (2),passivating metal (3), and noble metal (1) may be optimized to minimizethe quantity of precious metals required while simultaneously maximizingthe surface catalytic activity and providing an amount of thepassivating metal sufficient to produce a self-protecting nanoparticlealloy.

The passivating metal (3) forms a stable bond with one or moreconstituents of the environment in which the nanoparticle alloy istypically used as a catalyst. This bond effectively passivates surfaceregions of the nanoparticle in FIG. 1A, forming an impervious layerwhich physically shields exposed non-noble metal areas of the underlyingcore from any corrosive environment to which the nanoparticle may beexposed. This passivating layer forms essentially instantaneously overexposed non-noble metal areas due to the strong affinity for forming astable chemical bond with the passivating metal. The process isanalogous to the passivation of Si(001) surfaces during semiconductorlithography by immersion in hydrofluoric acid to form a H-terminatedsurface which is impervious to oxidation in the ambient. Consequently ananoparticle alloy may comprise at least one passivating element foundwithin columns IVB through VIIB of the periodic table.

The self-protecting nanoparticle alloys are not limited to homogeneoussolid solutions, but may be inhomogeneous. That is, the nanoparticlealloy may not have an even concentration distribution of each elementthroughout the nanoparticle itself. There may be precipitated phases,immiscible solid solutions, concentration nonuniformities, and somedegree of surface segregation. Furthermore, the self-protectingnanoparticle alloys may be formed using any suitable process aspreviously described. The key aspect is that the passivating metal ispresent in sufficient quantities near the surface of the nanoparticlealloy to form an impervious surface layer. When used in application suchas fuel cells, the improved stability of self-protecting alloys prolongsthe service life and improves the performance of the fuel cell. Theself-protecting nanoparticle alloys as described above are not limitedto fuel cells, but may also be used in any application wherenanoparticle catalysts are exposed to a corrosive environment.

B. Core-Shell Nanoparticles

In another embodiment, the nanostructured substrate may take the form ofa non-noble transition metal core which is covered with a thin film of anoble metal. The noble metal shell protects the underlying non-noblecore from corrosion during exposure to the acid-based electrolytes usedin subsequent processing steps and enables UPD of the intermediary metal(e.g., Cu) during subsequent deposition of a catalytically activeoverlayer as detailed in Section II below. The combination of core andshell metals used may also be suitably selected to enhance the catalyticproperties of the catalytic overlayer. This enhancement may beaccomplished by electronic effects and/or by adjusting the latticeparameter of the surface shell to induce strain in the overlayer suchthat its catalytic activity increases.

Core-shell nanoparticles may be formed using, for example, processesdescribed by J. Zhang, et al. in “Platinum Monolayer on NonnobleMetal-Noble Metal Core-Shell Nanoparticle Electrocatalysts for O₂Reduction,” J. Phys. Chem. B. 105, 22 701 (2005) (hereinafter “Zhang”)and U.S. Patent Publ. No. 2007/0031722 to Adzic, et al. the entirecontents of both of which are incorporated by reference as if fully setforth in this specification. Initially, a nanoparticle core comprised ofa non-noble transition metal such as, for example, nickel (Ni), cobalt(Co), or iron (Fe) along with a noble metal is formed. The non-noblecore metals may be used either alone or alloyed with other non-nobletransition metals. A core-shell system may be formed from nanoparticlescomprising, for example, Ni—Au, Co—Pd, or Co—Pt alloys. Subsequentelevated temperature annealing of nanoparticles formed of these alloysdrives surface segregation of the noble metal (e.g., Au, Pd, or Pt).This results in a nanoparticle comprising a non-noble metal coresurrounded by a noble metal shell. In another embodiment a nanoparticlecore comprised of a single non-noble metal may initially be formed usingany of the powder-forming processes detailed above. The non-noble metalcore is not limited to Ni, Co, or Fe, but also may be a refractory metal(i.e., W, Mo, Nb, Ta, or Rh). This core may then be covered with a thinshell of Pd, Au, Ru or another noble metal by a suitable process such aselectroless deposition or by chemical routes such as atomic layerdeposition (ALD) or CVD. An example of a core-shell nanoparticle isshown in FIG. 1B, where a core of a non-noble metal (2) is covered by anoble metal shell (4). The core-shell nanoparticle illustrated in FIG.1B also contains a passivating metal (3) from group IVB through VIIBwithin the core and its inclusion is described below.

In some instances the noble metal shell (4) may not completely cover theunderlying core. In this case, the non-noble metal (2) component of thecore may gradually erode due to reaction within a corrosive environment.By including an element from column IVB, VB, VIB, or VIIB (correspondingto groups 4 through 7, respectively) of the periodic table within eitherthe core, the shell, or both the core and shell, corrosion of thenanoparticle is inhibited. As described in Section A above withreference to nanoparticle alloys, the passivating metal (i.e., Ti, Hf,Zr, W, Ta, Nb, V, Re, Cr, Mo, Tc, or Mn) forms a chemical bond either atthe surface of the shell or at surface regions of the core which are notcovered by the shell. An example of a core-shell nanoparticle in which apassivating metal (3) has been incorporated with the noble metal shell(4) is shown in FIG. 1C. In FIG. 1D, an example in which the passivatingmetal (3) is alloyed with both the non-noble metal core (2) and noblemetal shell (4) is provided. The inclusion of a passivating element incore-shell particles produces a high-stability, self-protectingnanoparticle catalyst.

In addition to the surface segregation processes used by Zhang andAdzic, a shell of a noble metal or group IVB through VIIB element eitheralone or alloyed with one or more other transition metals may be formedby other aqueous or vapor-phase processes. For example, a film having ananoscale thickness (e.g., a nanofilm) may be formed on nanoparticles bya simple electroless deposition process from nonaqueous solutions. Thefilm thickness can be increased by additional conventional electrolessdeposition processes. Alternatively, an atomically thick shell layer(see, e.g., FIG. 1C) comprising a group IVB, VB, VIB, or VIIB metal maybe formed from vapor phase processes such as ALD, CVD, pulsed laserdeposition (PLD), or even physical vapor deposition (PVD) techniquessuch as sputtering, e-beam evaporation, or MBE. In another embodimentthe shell may be formed by a cation-adsorption-reduction-metaldisplacement method as detailed in Section II below. In still anotherembodiment an oxide nanoparticle of a group IVB, VB, VIB, or VIIB metalmay be mixed with PdCl₂ and active carbon then reduced in H₂ at elevatedtemperatures to produce alloys with surface-segregated Pd layers.

As is the case for nanoparticle alloys, the core-shell nanoparticles maybe homogeneous or have their constituents distributed nonuniformly. Thekey aspect is the presence of a group IVB, VB, VIIB, VIIB metalcomponent around the periphery of the nanoparticle such that apassivating surface layer can be formed in areas where a reactivenon-noble metal is exposed. Since this passivating layer formsspontaneously, the core-shell nanoparticle is self-protecting and itsstability is significantly increased.

It is to be understood that the methods of forming self-protectingnanoparticles as described above are merely exemplary; a plurality ofalternate methods may be employed. The desired composition, structure,and size range may be obtained via suitable adjustment of the processingparameters.

II. Deposition of a Catalytically Active Thin Film

Nanoparticle formation is followed by the deposition of a catalyticallyactive surface layer having thicknesses in thesubmonolayer-to-multilayer range. For purposes of this specification, amonolayer (ML) is formed when the surface of a substrate is fullycovered by a single, closely packed layer comprising adatoms of a secondmaterial which forms a chemical or physical bond with atoms at thesurface of the substrate. The surface is considered fully covered whensubstantially all available surface sites are occupied by an adatom ofthe second material. If the surface of the substrate is not completelycovered by a single layer of the adsorbing material, then the surfacecoverage is considered to be submonolayer. However, if a second orsubsequent layers of the adsorbant are deposited onto the first layer,then multilayer surface coverages (e.g., bilayer, trilayer, etc.)result.

The catalytically active surface layer may be deposited using any of awide variety of thin film deposition processes which are well-known inthe art. These include, but are not limited to, thermal evaporation,chemical vapor deposition (CVD), MBE, PLD, sputtering, and atomic layerdeposition (ALD). A majority of these techniques require specializedequipment capable of attaining medium to ultrahigh vacuum conditions andproviding precise control over the impinging flux of atoms. Thus, thesedeposition techniques tend to be prohibitively expensive.

Electrodeposition, on the other hand, is a robust, relatively low-costdeposition technique capable of controllably depositing thin films withthicknesses ranging from submonolayer coverages up to several microns.Electrodeposition may be carried out in aqueous or nonaqueous solutionsas well as solutions comprising an ionic liquid. Within thisspecification it is to be understood that the terms electrodepositionand electroplating may be used interchangeably with each referring tothe use of an electrochemical redox reaction to deposit a solid metalliccomposition onto a substrate from an aqueous or non-aqueous solution.The metallic composition itself may be deposited from a solutioncomprising a metal ion or a plurality of metal ions using methodswell-known to those skilled in the art.

A new synthetic procedure which employs the principles ofelectrodeposition and galvanic displacement has been utilized byBrankovic, et al. (hereinafter “Brankovic”) to deposit a monolayer of Ptonto Au(111) substrates and by Adzic, et al. (hereinafter “Adzic”) todeposit Pt monolayers onto Pd(111) and carbon-supported Pdnanoparticles. These procedures are described, for example, in “MetalMonolayer Deposition By Replacement Of Metal Adlayers On ElectrodeSurfaces,” Surf. Sci., 474, L173 (2001) and U.S. Patent Publ. No.2006/0135359, respectively. This process has also been described indetail by J. Zhang, et al. in “Platinum Monolayer Electrocatalysts forO₂ Reduction: Pt Monolayer On Pd(111) And On Carbon-Supported PdNanoparticles,” J. Phys. Chem. B108, 10955 (2004). Each of theaforementioned references is incorporated by reference as if fully setforth in this specification.

The deposition process is centered around a series of electrochemicalreactions which, when performed sequentially result in a film with thetargeted coverage and composition. The procedure involves the initialformation of an adlayer of a metal onto a substrate by underpotentialdeposition (UPD). This is followed by the galvanic displacement of theadlayer by a more noble metal, resulting in the conformal deposition ofa ML of the more noble metal on the substrate. The overall processinvolves the irreversible and spontaneous redox displacement of anadlayer of a non-noble metal by a more noble metal. This enables thecontrolled deposition of a thin, continuous layer of a desired metal.The process requires that the substrate metal be more noble than themetal undergoing deposition in order to avoid becoming oxidized. Theredox reaction can be described by the following equation

M_(UPD) ⁰+(m/z)L^(z+)

M^(m+)+(m/z)L⁰  (1)

where M_(UPD) ⁰ represents a UPD metal adatom on the electrode surfaceand L^(z+) is a noble metal cation with positive charge z+ and valencez. The M^(m+) represents the metal cation in the solution obtained afterthe UPD adatom was oxidized, and L⁰ is a noble atom deposited in theredox process.

Although the catalytically active surface layer is not limited to anyparticular material, it is preferably Pt due to its excellent catalyticproperties. Consequently, an example in which a monolayer of Pt isformed on nanoparticles using the processes described by Brankovic andAdzic will now be described in detail. It is to be understood, however,that the process is not limited to Pt and other noble metals may beutilized. The method involves the initial formation of a monolayer of ametal such as copper (Cu) by underpotential deposition (UPD) in asolution comprised of 50 mM CuSO₄ in a 50 mM H₂SO₄ solution. TheCu-coated nanoparticles are then emersed from solution and rinsed withdeionized water to remove Cu²⁺ ions from the surface. This is followedby immersion in a solution comprised of 1.0 mM K₂PtCl₄ in 50 mM H₂SO₄under a N₂ atmosphere for approximately two minutes to replace all Cuatoms with Pt atoms. The Pt-coated nanoparticle substrate is againrinsed with deionized water. The above processes are carried out in amulti-compartment cell under a N₂ atmosphere in order to prevent Cuoxidation by O₂ during sample transfer.

The above process results in the conformal deposition of a ML of Pt onhigh-surface-area nanoparticle alloys or core-shell nanoparticles. Thedeposition cycle comprising UPD of Cu followed by galvanic displacementwith Pt may be repeated as needed to produce two or more layers of Pt inorder to ensure complete coverage of the nanoparticle surface.Conversely, the UPD of Cu may be controllably limited such thatsubmonolayer coverages of Cu and, hence, Pt are obtained. Deposition ofan initial adlayer by UPD may also be accomplished using metals otherthan Cu such as, for example, lead (Pb), bismuth (Bi), tin (Sn), cadmium(Cd), silver (Ag), antimony (Sb), and thallium (Tl). The choice of metalused for UPD will influence the final Pt surface coverage obtained for agiven UPD adlayer. This occurs due to variations in the size and valencyamong the different metals. The metal overlayer used is not limited toPt, but may be formed from other noble metals with the only requirementbeing that the desired metal be more noble than the UPD adlayer. Thismay be accomplished by contacting the copper-coated particles with theircorresponding salts. For example, monolayers of iridium, ruthenium,osmium, and rhenium can be deposited by displacement of a ML of a lessnoble metal such as copper using IrCl₃, RuCl₃, OsCl₃, or ReCl₃,respectively. Furthermore, the metal overlayer may be formed as an alloywith any number of constituents such as binary, ternary, quaternary, or,quinary alloys with experimentally optimized stoichiometry ratios.

The process offers unprecedented control over film growth and isadvantageous in terms of its versatility, reproducibility, and efficientutilization of source material. Since a costly precious metal such as Ptcan be utilized as a thin film instead of in bulk form, significant costsavings can be attained. The utilization of a noble metal/substratenanoparticle may also provide unexpectedly heightened catalytic activitydue to synergistic effects between the nanoparticles and the catalyticoverlayer. The unexpected increase in catalytic activity may arise dueto electronic and geometric effects which arise from the formation ofsurface metal-metal bonds and the differing lattice constants of thecatalytic overlayer and underlying substrate.

The catalytic properties of the surface overlayer may also be engineeredby use of a suitable core-shell nanoparticle. A core of a non-noblemetal such as Ni, Co, Fe, Ti, W, Nb, V, or Ta may be coated with a morenoble metal such as Au, Pd, or Pt. The catalytic activity of the finalcoated nanoparticle may be controlled by engineering the electronicproperties and lattice parameter of the underlying core-shellnanoparticles with respect to those of the metal overlayer.

A first general embodiment describing a method of forming a Pt overlayeron the surface of a self-protecting nanoparticle alloy will now bedescribed in detail with reference to FIGS. 2-4. Specific embodimentswhich exemplify electrocatalyst particles comprising at least onepassivating element are provided following the general embodiment inExamples 1 and 2 below. The particle size, microstructure, and activityare analyzed and the results are provided in FIGS. 5-11. Example 1discloses nanoparticles of a Pd₃Ti alloy whereas Example 2 is directedto PdRe alloy nanoparticles. It is to be understood that theseembodiments are merely exemplary and are used to describe a preferredmode of practicing the invention: It is to be further understood thatthere are many other possible variations which do not deviate from thespirit and scope of the present invention.

III. Exemplary Embodiments

An exemplary embodiment of the present invention will now be describedin detail with reference to FIG. 2 which shows a sequence of surfacechemical reactions culminating in the formation of a Pt shell on ahigh-stability, self-protecting nanoparticle alloy surface. The desirednanoparticles are initially formed using any of the plurality of methodsdescribed in Section I above. For purposes of this embodiment, only thefirst two surface atomic layers of a self-protecting nanoparticle alloyare shown in FIG. 2. The nanoparticle surface in FIG. 2 comprises noblemetal atoms (1), a non-noble metal component (2), and a passivatingelement (3).

Non-noble metal ions of Cu²⁺ (5) are initially adsorbed on the surfaceby immersing the nanoparticles in a plating bath comprising theappropriate concentration of Cu²⁺ ions (5) in step S1. UPD of Cu resultsin the adsorption of Cu²⁺ ions (5) on the nanoparticle surface in stepS2 and the formation of a monolayer of Cu (6) in step S3. This monolayerforms a continuous Cu “skin” around the periphery of the nanoparticle.The nanoparticle is them emersed from the bath and rinsed with deionizedwater to remove excess Cu²⁺ (5) ions on the surface. The sample ismaintained under a N₂ atmosphere during transfer to inhibit oxidation ofthe freshly deposited Cu adlayer (6). The nanoparticle is then immersedin a solution comprising a Pt salt in step S4 where Pt²⁺ ions (7)replace surface Cu adatoms (6) via a redox reaction as illustrated instep S5. Since Pt is more noble than Cu, it acts as an oxidizing agentby accepting electrons from Cu. The simultaneous reduction of Pt²⁺ ionsto Pt (8) results in the replacement of surface Cu atoms (6) with Ptatoms (8). The final product is a Pt nanoparticle with a “skin”comprising a monolayer of Pt atoms in step S6.

An illustration of a Pt-encapsulated self-protecting nanoparticle alloyand a Pt-encapsulated core-shell nanoparticle are provided in FIGS. 3and 4, respectively. The cross-section shows that all atoms areclose-packed in a hexagonal lattice, resulting in a hexagonal shape. Itis to be understood, however, that the crystallographic structure is notlimited to that shown and described in FIGS. 3 and 4. Furthermore, theratio of noble metal (1) and (4), non-noble metal (2), and passivating(3) atoms illustrated in FIGS. 2-4 was arbitrarily chosen to illustratethe principles of the invention. The cycle depicted in FIG. 2 may berepeated any number of times to deposit additional layers of Pt onto thesurface of the nanoparticle to ensure complete coverage. Conversely,less than a monolayer of Cu may be deposited during UPD such thatsubmonolayer coverages of Pt result. While only a portion of the surfaceof a single nanoparticle is illustrated in FIG. 2 it is to be understoodthat Pt deposition will simultaneously occur on a large number ofnanoparticles. The “skin” of Pt atoms will form a continuous andconformal coverage of the entire available surface area.

Example 1

In another embodiment, carbon-supported Pd—Ti nanoparticle alloys wereprepared by dissolving TiCl₄(OC₅H₁₀)₂ powder in dimethyl ether (DME).The resulting solution is mixed with Pd(acac)₂, a thiol, and carbonpowder at room temperature. The nominal ratio of Pd to Ti is set as 3:1in order to produce Pd₃Ti/C nanoparticle alloys. The mixture is thensonicated, stirred at room temperature for two hours, and then driedunder an atmosphere of H₂ gas. The resulting powder was then transferredto an oven where it was heated to 900° C. in an Ar/H₂ atmosphere for twohours and cooled to room temperature while maintaining a continuousAr/H₂ flow. The microstructure of the resulting Pd₃Ti/C nanoparticleswas examined by transmission electron microscopy (TEM) and a samplemicrograph is provided in FIG. 5. The carbon support is illustrated inFIG. 5 as the lighter-colored background material whereas the Pd₃Tinanoparticles appear as comparatively darker-colored particles whichappear hexagonal in cross-section. The Pd₃Ti particle size ranges from 7to 30 nm with the average particle size being approximately 20 nm. TEMresults also reveal that the particles themselves are substantially inthe shape of a cuboctahedron bound predominantly by (111) and (100)planes.

X-ray diffraction (XRD) analyses of the Pd₃Ti nanoparticles between2θ=30° and 60° (see, e.g., the upper curve in FIG. 6) show relativelynarrow (111) and (200) peaks at 2θ˜40° and 47°. An XRD scan obtainedfrom commercial Pd/C is provided as the bottom curve in FIG. 6 forcomparison. Compared to Pd nanoparticles (lower curve), Pd₃Tinanoparticle alloys exhibit sharper and more well-defined Pd(111) andPd(200) crystalline peaks which are shifted to higher 2θ values. Thissuggests that, compared to Pd nanoparticles, the Pd₃Ti nanoparticlealloys have a more well-defined crystal structure and sharper surfaceplanes.

A platinum monolayer was deposited onto the Pd₃Ti nanoparticles by redoxdisplacement by platinum of an adlayer of an underpotentially deposited(UPD) metal. In this example, Cu was used as the UPD metal on thePd₃Ti/C nanoparticle substrate. To prepare an electrode with Pd₃Tinanoparticles, a dispersion of carbon-supported Pd₃Ti nanoparticles(Pd₃Ti/C) on a carbon substrate was made by sonicating the Pd₃Ti/Cnanoparticles in water for about 5-10 minutes to make a uniformsuspension. The carbon substrate used was Vulcan XC-72. Then, 5microliters of this suspension was placed on a glassy carbon disc (GC)electrode and dried in air. The GC electrode holding the Pd₃Ti/Cnanoparticles was then placed in a 50 mM CuSO₄/0.1M H₂SO₄ solution toelectrodeposit Cu. After electrodeposition of a Cu ML, the electrode wasrinsed to remove Cu ions from the electrode. The electrode was thenplaced in an aqueous solution containing 1.0 mM K₂PtCl₄ in 50 mM H₂SO₄in a nitrogen atmosphere. After a 1-2 minute immersion to completelyreplace Cu by Pt, the electrode was rinsed again. The deposition of anatomic ML of Pt on Pd₃Ti nanoparticles was verified by voltammetry andAuger electron spectroscopy (AES). All of these operations were carriedout in a multi-compartment cell in a nitrogen atmosphere that preventsthe oxidation of Cu adatoms in contact with O₂.

The oxygen reduction electrocatalytic activity of Pt_(ML)/Pd₃Ti/Cnanoparticle composites was compared to the electrocatalytic activity ofcommercially available TEC10E50E 46.4% Pt on Pt nanoparticle catalystsby measuring polarization curves using a rotating disc electrode in aroom-temperature solution of 0.1 M HClO₄, a scan speed of 10 mV/s, and arotation speed of 1600 rpm. Experimental results are shown in FIG. 7,which provides electrocatalytic oxygen reduction curves obtained fromPt/Pd₃Ti/C and commercial Pt nanoparticle catalysts. The activity of thePt ML on Pd₃Ti nanoparticles is slightly higher than that of Ptnanoparticles. Pt loading for the Pt_(ML)/Pd₃Ti/C nanoparticles is 7μg_(Pt)/cm² whereas for the commercial Pt/C nanoparticles it is 21μg_(Pt)/cm².

A comparison of the mass-specific activities of Pt_(ML)/Pd₃Ti/C and Pt/Celectrocatalysts is displayed in FIG. 8 expressed as the kinetic currentj_(k) in milliamps per microgram (mA/μg) at 0.90 V divided by the Ptmass. The kinetic current j_(k) provides a measure of the activity ofthe nanoparticles per unit mass of Pt that is included in thenanoparticles. Thus, the higher the value of j_(k), the larger thecatalytic activity attained per unit mass of Pt. The electrode having PtML particles (Pt_(mL)/Pd₃Ti/C) has a 3.5 to 4.5 times highermass-specific activity than the electrode with Pt nanoparticles.

Example 2

In yet another embodiment, carbon-supported Pd—Re nanoparticle alloys(PdRe/C) were prepared in a manner analogous to that described inExample 1. The PdRe/C nanoparticles were prepared by dissolvingReCl₄(OC₅H₁₀)₂ powder in dimethyl ether (DME). The resulting solution ismixed with Pd(acac)₂, a thiol, and carbon powder at room temperature.The ratio of Pd to Re is set as 1:1 in order to produce nanoparticlealloys having equal amounts of Pd and Re. The mixture is then sonicated,stirred at room temperature for two hours, and then dried under anatmosphere of H₂ gas. The resulting powder was then transferred to anoven where it was heated under an H₂ atmosphere to either 800° C. or600° C. for three hours and then cooled to room temperature whilemaintaining a continuous H₂ flow.

X-ray diffraction (XRD) analyses of the PdRe/C nanoparticles andcommercial Pd/C nanoparticles were obtained over the range 2θ=30° to 70°and the resulting spectra are provided in FIG. 9. The top and middlecurves were obtained from separate batches of PdRe nanoparticle alloysannealed at 800° C. and 600° C., respectively, for 3 hours each. An XRDscan obtained from commercial Pd/C is provided as the bottom curve inFIG. 9 for comparison. As was the case for Pd₃Ti nanoparticle alloys,compared to Pd nanoparticles, PdRe nanoparticle alloys exhibit sharperand more well-defined crystalline peaks which are shifted to slightlyhigher 2θ values. This again suggests that, compared to Pdnanoparticles, the PdRe nanoparticle alloys have a more well-definedcrystal structure and sharper surface planes. FIG. 9 shows that the PdRenanoparticles also exhibit diffraction peaks arising from Re(001),Re(002), Re(101), and Re(110) lattice planes.

A platinum monolayer was also deposited onto the PdRe nanoparticles byredox displacement by platinum of an adlayer of an underpotentiallydeposited (UPD) metal. The process followed is identical to thatdescribed in Example 1 and will be omitted for brevity. The oxygenreduction electrocatalytic activity of Pt_(ML)/PdRe/C nanoparticlecomposites was also compared to the electrocatalytic activity ofcommercially available TEC10E50E 46.4% Pt on Pt nanoparticle catalystsby measuring polarization curves using a rotating disc electrode in aroom-temperature solution of 0.1 M HClO₄, a scan speed of 10 mV/s, and arotation speed of 1600 rpm. A comparison of the mass-specific activitiesof Pt_(ML)/PdRe/C and Pt/C electrocatalysts is displayed in FIG. 10expressed as the kinetic current j_(k) in milliamps per microgram(mA/μg) at 0.90 V divided by the Pt mass. As was the case forPt_(ML)/Pd₃Ti/C nanoparticle alloys, the Pt_(ML)/Pd₃Ti/C electrode has a3.5 to 4.5 times higher mass-specific activity than the electrode withPt nanoparticles.

The stability of Pt_(ML)/PdRe/C nanoparticles was investigated bymeasuring the polarization curves before and after performing repeatedpotential cycles between 0.5 and 0.95 V. The results are provided inFIG. 11 and show that there is essentially no change in the half wavepotential or the overall shape of the polarization curve afterperforming 10,000 potential cycles. These results indicate that, whencompared to a commercial Pt nanoparticle electro catalyst, aPt-encapsulated nanoparticle core comprising a passivating elementexhibits significant improvements in both the catalytic activity andstability.

IV. Energy Conversion Devices

In a preferred application, the Pt-coated self-protecting nanoparticlesas described above may be used as an electrode in a fuel cell. In theevent the Pt surface coverage is incomplete, the passivating element (3)present in the nanoparticle may form a stable bond with select elementsor compounds from the environment in which is used. This blocks accessto the non-noble metal (2) core constituents, thereby inhibitingcorrosion of the electrocatalyst nanoparticle support. This applicationis, however, merely exemplary and is being used to describe a possibleimplementation of the present invention. Implementation as a fuel cellelectrode is described, for example, in U.S. Patent Puble. No.2006/0135,359 to Adzic. It is to be understood that there are manypossible applications which may include, but are not limited to H₂sensors, charge storage devices, applications which involve corrosiveprocesses, as well as various other types of electrochemical orcatalytic devices.

A schematic showing an example of a fuel cell and its operation isprovided in FIG. 12. A fuel such as hydrogen gas (H₂) is introducedthrough a first electrode (10) whereas an oxidant such as oxygen (O₂) isintroduced through the second electrode (11). In the configuration shownin FIG. 12, the first electrode (10) is the anode and the secondelectrode (11) is the cathode. At least one electrode is comprised ofPt-coated core-shell nanoparticles which, in a preferred embodiment,have a non-noble core coated with a shell of a noble metal. Understandard operating conditions electrons and ions are separated from thefuel at the anode (10) such that the electrons are transported throughan external circuit (12) and the ions pass through an electrolyte (13).At the cathode (11) the electrons and ions combine with the oxidant toform a waste product which, in this case, is H₂O. The electrical currentflowing through the external circuit (12) can be used as electricalenergy to power conventional electronic devices. The increase in the ORRattainable through incorporation of Pt-coated core-shell nanoparticlesin one or more electrodes will produce an increase in the overall energyconversion efficiency of the fuel cell. Consequently, for a givenquantity of fuel, a larger amount of electrical energy will be producedwhen using Pt-coated core-shell nanoparticle electrodes compared toconventional nanoparticle electrodes.

It will be appreciated by persons skilled in the art that the presentinvention is not limited to what has been particularly shown anddescribed in this specification. Rather, the scope of the presentinvention is defined by the claims which follow. It should further beunderstood that the above description is only representative ofillustrative examples of embodiments. For the reader's convenience, theabove description has focused on a representative sample of possibleembodiments, a sample that teaches the principles of the presentinvention. Other embodiments may result from a different combination ofportions of different embodiments.

The description has not attempted to exhaustively enumerate all possiblevariations. The alternate embodiments may not have been presented for aspecific portion of the invention, and may result from a differentcombination of described portions, or that other undescribed alternateembodiments may be available for a portion, is not to be considered adisclaimer of those alternate embodiments. It will be appreciated thatmany of those undescribed embodiments are within the literal scope ofthe following claims, and others are equivalent. Furthermore, allreferences, publications, U.S. patents, and U.S. patent applicationPublications cited throughout this specification are incorporated byreference as if fully set forth in this specification.

1. An electrocatalyst comprising: a particle comprising an alloy formedwith at least one element selected from the group consisting of columnsIVB, VB, VIB, and VIIB of the periodic table; and an atomically thinlayer of platinum atoms at least partially encapsulating the particle.2. The electrocatalyst of claim 1 wherein the particle comprises anelement selected from the group consisting of Ti, Hf, Zr, W, Ta, Nb, V,Re, Cr, Mo, Tc, and Mn.
 3. The electrocatalyst of claim 1 wherein theparticle comprises a binary metal alloy comprising a transition metaland a passivating metal according to the formula Tr_(1-x)Ps_(x) where Tris the transition metal and Ps is the passivating metal and x representsthe concentration of Ps, being adjustable over the range 0<x<1.
 4. Theelectrocatalyst of claim 1 wherein the particle comprises an alloyselected from the group consisting of Pd_(1-x)Ti_(x), Pd_(1-x)W_(x),Pd_(1-x)Nb_(x), Pd_(1-x)Ta_(x), Pd_(1-x)Re_(x), Pd_(1-x)Ir_(x),Ir_(1-x)Ti_(x), Ir_(1-x)Ta_(x), Ir_(1-x)Nb_(x), Ir_(1-x)Re_(x),Au_(1-x)Ta_(x), Au_(1-x)Ir_(x), and Au_(1-x)Re_(x) and x represents theconcentration of the alloying element, being adjustable over the range0<x<1.
 5. The electrocatalyst of claim 1 wherein the particle comprisesa noble metal.
 6. The electrocatalyst of claim 5 wherein the particlefurther comprises a non-noble metal.
 7. The electrocatalyst of claim 1wherein the atomically thin layer of platinum atoms is one to threemonolayers thick.
 8. The electrocatalyst of claim 1, wherein theparticle is a nanoparticle having dimensions of 1 to 100 nm along threeorthogonal directions.
 9. The electrocatalyst of claim 1, wherein theparticle is spherical.
 10. An electrocatalyst comprising: a core atleast partially encapsulated by a shell to form a core-shell particle inwhich the core and shell have different compositions; and an atomicallythin layer of platinum atoms at least partially encapsulating theparticle; wherein at least one of the core or shell is comprised of analloy formed with at least one element selected from the groupconsisting of columns IVB, VB, VIB, and VIIB of the periodic table. 11.The electrocatalyst of claim 10 wherein at least one of the core orshell comprises an element selected from the group consisting of Ti, Hf,Zr, W, Ta, Nb, V, Re, Cr, Mo, Tc, and Mn.
 12. The electrocatalyst ofclaim 10 wherein the particle comprises a binary metal alloy comprisinga transition metal and a passivating metal according to the formulaTr_(1-x)Ps_(x) where Tr is the transition metal and Ps is thepassivating metal and x represents the concentration of Ps, beingadjustable over the range 0<x<1.
 13. The electrocatalyst of claim 10wherein the particle comprises an alloy selected from the groupconsisting of Pd_(1-x)Ti_(x), Pd_(1-x)W_(x), Pd_(1-x)Nb_(x),Pd_(1-x)Ta_(x), Pd_(1-x)Re_(x), Pd_(1-x)Ir_(x), Ir_(1-x)Ti_(x),Ir_(1-x)Ta_(x), Ir_(1-x)Nb_(x), Ir_(1-x)Re_(x), Au_(1-x)Ta_(x),Au_(1-x)Ir_(x), and Au_(1-x)Re_(x) and x represents the concentration ofthe alloying element, being adjustable over the range 0<x<1.
 14. Theelectrocatalyst of claim 10 wherein the core comprises a non-noblemetal.
 15. The electrocatalyst of claim 14 wherein the shell comprises anoble metal.
 16. The electrocatalyst of claim 10 wherein the atomicallythin layer of platinum atoms is one to three monolayers thick.
 17. Theelectrocatalyst of claim 10, wherein the particle is a nanoparticlehaving dimensions of 1 to 100 nm along three orthogonal directions. 18.The electrocatalyst of claim 10 wherein the core-shell particle isspherical.
 19. A method of forming electrocatalyst particles comprising:forming particles comprising a predetermined ratio of atoms of atransition metal and at least one metal selected from the groupconsisting of columns IVB, VB, VIB, and VIIB of the periodic table;depositing a contiguous non-noble metal adlayer on a surface of theparticles; and replacing the contiguous non-noble metal adlayer with anoble metal.
 20. The method of claim 19 wherein the particle is ananoparticle having dimensions of 1 to 100 nm along three orthogonaldirections.
 21. The method of claim 20 wherein the nanoparticles areformed with at least one metal selected from the group consisting of Ti,Hf, Zr, W, Ta, Nb, V, Re, Cr, Mo, Tc, and Mn.
 22. The method of claim 19wherein the particles are formed by dissolving TiCl₄(OC₅H₁₀)₂ powder indimethyl ether (DME) and mixing the resulting solution with Pd(acac)₂, athiol, and carbon powder at room temperature.
 23. The method of claim 22wherein the ratio of Pd to Ti is 3:1.
 24. The method of claim 22 whereinthe particles are sonicated, stirred at room temperature for two hours,and then dried under an H₂ atmosphere.
 25. The method of claim 24wherein the particles are heated to 900° C. in an Ar/H₂ atmosphere fortwo hours and cooled to room temperature while maintaining a continuousAr/H₂ flow.
 26. The method of claim 19 wherein the particles are formedby dissolving ReCl₄(OC₅H₁₀)₂ powder in dimethyl ether (DME) and mixingthe resulting solution with Pd(acac)₂, a thiol, and carbon powder atroom temperature.
 27. The method of claim 26 wherein the ratio of Pd toRe is 1:1.
 28. The method of claim 26 wherein the particles aresonicated, stirred at room temperature for two hours, and then driedunder an H₂ atmosphere.
 29. The method of claim 28 wherein the particlesare heated to 600° C. in an H₂ atmosphere for three hours and cooled toroom temperature while maintaining a continuous H₂ flow.
 30. The methodof claim 28 wherein the particles are heated to 800° C. in an H₂atmosphere for three hours and cooled to room temperature whilemaintaining a continuous H₂ flow.
 31. The method of claim 20 wherein thenanoparticles are formed by reducing an aqueous suspension comprising apredetermined ratio of a non-noble metal salt and a salt of at least onemetal selected from the group consisting of columns IVB, VB, VIB, andVIIB of the periodic table to form particles comprising atoms of anon-noble metal and at least one metal selected from the groupconsisting of columns IVB, VB, VIB, and VIIB.
 32. The method of claim 31further comprising annealing the nanoparticles to form core-shellnanoparticles comprising a non-noble metal core and a shell comprisingat least one metal selected from the group consisting of columns IVB,VB, VIB, and VIIB of the periodic table.
 33. The method of claim 31further comprising forming a shell of a noble metal on the thus-formednanoparticles.
 34. The method of claim 20 wherein the nanoparticles areformed by ball milling, atomization of molten metal forced through anorifice at high velocity, centrifugal disintegration, sol-gelprocessing, or by vaporization of a liquid metal followed bysupercooling in an inert gas stream.
 35. The method of claim 19 whereinthe contiguous non-noble metal adlayer is deposited by underpotentialdeposition.
 36. The method of claim 35 wherein the contiguous non-noblemetal adlayer is selected from the group consisting of Cu, Pb, Bi, Sn,Cd, Ag, Sb, and Tl.
 37. The method of claim 19 wherein the contiguousnon-noble metal adlayer is replaced by a noble metal by immersing theparticles comprising a salt of a noble metal.
 38. The method of claim 37wherein the noble metal salt consists of K₂PtCl₄.